A traditional camera system includes camera body, lens, electronics to control the functionality of the camera and most importantly the image-capturing media—film. In contrast, instead of using film, a digital camera system usually employs an image sensor made on semiconductor substrate. Typically the image sensor is either a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor. They can be either in two-dimensional array called area sensor or one-dimensional array called linear sensor. For illustrative purpose, the image sensor discussed in this patent is an area CCD image sensor.
An area CCD image sensor consists of a two-dimensional array of photosensitive elements called pixels of X columns by Y rows. These pixels capture light from a scene of interest during exposure and converts photons to electrical charge, namely electrons. The electrons are then transferred out of the CCD vertically and horizontally. Each CCD has at least one output structure or amplifier, which consists of one or more transistors to convert the electrons from the pixels to voltage signals. These voltage signals are then sampled and converted to digital counts through a correlated double sampling (CDS) and analog-to-digital converter (ADC). Finally a digital image processor processes these digital counts and the counts are rendered as a digital image.
An amplifier typically consists of transistors, each of which has a drain, a source and a gate. Referring to FIG. 1A, there is shown a symbol to represent a transistor. Also referring to FIG. 1B there is shown a cross-section of an NMOS or n-channel transistor 1 built on a p-type silicon layer 5. It is noted that other types of transistors are applicable to the present invention, in which case the doping will vary according, as those skilled in the art will readily recognize. The source 2 and drain 3 of the transistor 1 consist of n-type doping regions. The voltage at contact 6 of the source 2 is modulated by the voltage on the gate 4 when the transistor 1 is operated in a source-follower amplifier configuration. The drain 3 of the transistor 1 in this amplifier configuration is connected to a high-voltage applied to the contact 7, commonly referred to as VDD. This high voltage at drain 3 will create a strong electric field between the gate 4 and the drain 3 that may generate near-infra-read (NIR) light. This NIR light can propagate inside the silicon or radiate outside and over the silicon. If the light or electrons created thereby eventually enter the image area of the CCD, it will cause a spurious signal in the image area. These additional charges will superimpose with the existing charge related to the scene of interest and create a glowing phenomenon or artifact seen in one of the corners of the CCD where the amplifier is located.
Referring to FIG. 2 there is shown a diagram of a CCD 10 which has an amplifier 13 at the top-left corner. Charge is first transferred vertically row by row into a serial of horizontal register 14. The vertical transfer direction is indicated by an arrow 15. After each row of transfer, charge in the horizontal registers 14 is then transferred horizontally to the output amplifier 13. The horizontal transfer direction is indicated by an arrow 16. The light emitting from the amplifier 13 enters some portions of the top-left corner of image area 12 and creates so-called amplifier glowing artifact in that region 12 of the whole image area 11. The extra signal from the glowing will contaminate the image in that region if not removed or substantially reduced.
Referring to FIG. 3 there is shown a cross section of an amplifier transistor 20 in a CCD image sensor 28. It includes a source 21, a drain 22 and a gate 23. The source 21 and drain 22 typically consist of n-type doping and they are connected to metal contacts 24 and 34, respectively. Light mainly generates along the gate-drain side 25 and it propagates in every direction as indicated by the arrows. Among them there are three paths which may potentially impact the image area 12. One is the path 30 through which light strikes onto top layers (not shown) over the silicon surface 29 including oxides, protection layers and a cover glass (not shown) etc, and then bounces back into the image area 12. Another one is the path 31 through which the light passes through and underneath the silicon surface 29 and within the p-type epi layer 26 and reaches the image area 12. Path 31 has another potential impact. The charge 33, namely electrons, generated by the light along the path 31 can diffuse into the image area 12 also. Path 32 has little impact because of two reasons. One reason is the light going deep into the p+-type substrate 27 will be either absorbed by the silicon or pass through and out of the substrate 27. There will be no light reflection into the image area 12 from path 32. The charge 33 in the substrate 27 generated by the absorbed light will recombine quickly because of its short diffusion length in that region. Therefore, there is virtually no charge diffusion into the image area 12 either. This is particularly true for devices built in a well or a tub with opposite conductivity type to the substrate in which this well is formed. In summary, paths 30 and 31 have the most significant impact on creating the glowing artifact in the image area 12.
Since an amplifier only glows when the VDD voltage is applied to it, the prior art solution to this problem is to minimize the time when the amplifier operates, which means during the exposure time, VDD is reduced or set to zero to turn off the amplifier. This method will remove the glowing during exposure, but the amplifier needs to be turned back on during CCD readout and in that period, the glowing will appear again and it will affect the images which the CCD captures. In this case, the glowing will cause a white band across the image. Therefore, the impact from the amplifier glowing cannot be completely removed by this method. Besides, switching VDD ON and OFF can sometimes generate unnecessary electronic noise in the system.
Therefore, there is a need to prevent the amplifier glowing from entering the image sensor's image area and creating a glowing artifact in the images which the image sensor captures.